A conventional inkjet printing system includes a printhead, an ink supply which supplies liquid ink to the printhead, and an electronic controller which controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles and toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as thin film resistors. Heating the ink causes the ink to vaporize and be ejected from the nozzles. To heat the ink, power is supplied to the thin film resistors. Power consumed by the thin film resistors is equal to Vi, where V is the voltage across the thin film resistor and i is the current through the thin film resistor. The electronic controller, which is typically located as part of the processing electronics of a printer, controls the power supplied to the thin film resistors from a power supply which is external to the printhead.
In one type of inkjet printing system, printheads receive fire signals containing fire pulses from the electronic controller. The electronic controller controls the drop generator energy of the printhead by controlling the fire signal timing. The timing related to the fire signal includes the width of the fire pulse and the point in time at which the fire pulse occurs. The electronic controller also controls the drop generator energy by controlling the electrical current passed through the thin film resistors by controlling the voltage level of the power supply.
Typically, control of the fire signal timing and the voltage level of the power supply works well for smaller printheads having smaller swath heights and for printheads capable of printing only a single color. These printheads tend to be relatively easier to control as they only need one fire signal to control the ejection of ink drops from the printhead.
With single color printheads having larger swath heights, thermal gradients can become pronounced. The thermal gradients can result in drop volume variation across the printhead. To offset this effect, the fire pulse width can be adjusted while printing using approaches such as dynamic pulse width adjustment (DPWA) algorithms. With large thermal gradients, there may not be a high enough degree of control in the DPWA algorithms to control the drop generator energy across the printhead.
Multiple color printheads which use black drop generators at higher drop volumes and color drop generators at lower drop volumes can also be difficult to control. Higher volume drop generators require a higher turn on energy than lower volume drop generators. Consequently, the ejection of ink drops from multiple color printheads can be difficult to control.
For reasons stated above and for other reasons presented in the Detailed Description section of the present specification, a fluid ejection device is desired which provides greater control of drop generator energy across the printhead.